An alloy rod having giant magnetostriction such as an amount of magnetostriction of, for example, at least 10.sup.-3 is now attracting the general attention as a material for elements of an electric audio converter, a vibrator, an actuator and the like, and industrialization thereof is under way. The alloy rod having such giant magnetostriction is usually manufactured by heat-treating a rod-shaped alloy material at a temperature slightly lower than the melting point thereof, or totally melting a granular or flaky alloy material and then solidifying the resultant melt of the alloy material into a rod shape.
An alloy comprising at least two rare earth metals including terbium (Tb) and dysprosium (Dy) and at least one transition metal is available as an alloy material for the alloy rod having such giant magnetostriction. As the above-mentioned alloy, an alloy having the following chemical composition is known in the product name of "Terfenol": EQU Tb.sub.X Dy.sub.Y Fe.sub.Z
where, X, Y and Z are ratios of the number of atoms, taking respectively the following values:
X: from 0.25 to 0.35, PA0 Y: from 0.60 to 0.80, and PA0 Z: from 1.5 to 2.0.
For the manufacture of an alloy rod having giant magnetostriction, the following methods are known:
(1) A method for manufacturing an alloy rod having giant magnetostriction, disclosed in Japanese Patent Provisional Publication No.53-64,798 dated June 9, 1978, which comprises the steps of:
subjecting a rod-shaped alloy material having a chemical composition comprising Tb.sub.0.28 Dy.sub.0.72 Fe.sub.2 to a heat treatment in an inert gas atmosphere, which heat treatment comprises heating said alloy material at a temperature slightly lower than the melting point thereof by means of an annular heater stationarily arranged so as to surround said alloy material while moving said alloy material in the axial direction thereof, thereby manufacturing an alloy rod having giant magnetostriction (hereinafter referred to as the "prior art 1").
(2) A method for manufacturing an alloy rod having giant magnetostriction, disclosed in Japanese Patent Provisional Publication No.62-109,946 dated May 21, 1987, which comprises the steps of:
receiving a rod-shaped alloy material having a chemical composition comprising Tb.sub.X Dy.sub.1-X Fe.sub.1.5-2.0 (X being from 0.27 to 0.35) into a chamber made of quartz; moving an annular high-frequency heating coil, arranged so as to surround said chamber, from the lower end toward the upper end of the chamber to heat said rod-shaped alloy material in said chamber in the circumferential direction thereof; continuously moving said heating from the lower end toward the upper end of said alloy material in the axial direction thereof to locally and sequentially melt said alloy material in the axial direction thereof; and then, locally and sequentially solidifying the resultant molten section of said alloy material in said chamber, thereby manufacturing an alloy rod having giant magnetostriction (hereinafter referred to as the "prior art 2").
(3) A method, based on the conventional Bridgman method, for manufacturing an alloy rod having giant magnetostriction, which comprises the steps of:
supplying a granular or flaky alloy material, comprising at least two rare earth metals including terbium and dysprosium and at least one transition metal, into a crucible in an inert gas atmosphere placed in a vertical cylindrical heating furnace; melting said alloy material in said crucible in said heating furnace; and then, downwardly moving said crucible in the vertical direction to solidify and crystallize the resultant melt of said alloy material in said crucible at a lower portion of said heating furnace, thereby manufacturing an alloy rod having giant magnetostriction (hereinafter referred to as the "prior art 3").
FIG. 6 is a schematic descriptive view illustrating a typical apparatus used in the method of the prior art 3. As shown in FIG. 6, a tubular crucible 21 made of ceramics is supported vertically in a cylindrical heating furnace 16 by means of a crucible support 18 on the upper end of a vertical crucible supporting shaft 19. The crucible supporting shaft 19 projects downwardly through a sealing member 20 from the lower end of the heating furnace 16, and is vertically movable by means of a driving mechanism not shown. The heating furnace 16 has an electric-resistance heater 17 arranged so as to surround the heating furnace 16. The upper end of the heating furnace 16 is capable of being opened and closed by means of a cover not shown.
A granular or flaky alloy material comprising at least two rare earth metals including terbium and dysprosium and at least one transition metal is supplied into the crucible 21. The crucible 21 supplied with the alloy material is positioned by means of the crucible supporting shaft 19 at a portion of the heating furnace 16, where the electric-resistance heater 17 is arranged. The alloy material in the crucible 21 is heated by means of the electric-resistance heater 17 of the heating furnace 16 to totally melt the alloy material. Then, the crucible 21 is moved by means of the crucible supporting shaft 19 to a level lower than the portion of the heating furance 16, where the electric-resistance heater 17 is arranged, to cool the crucible 21, thereby solidifying the resultant melt 24 of the alloy material in the crucible 21 to manufacture an alloy rod.
FIG. 7 is a schematic descriptive view illustrating another apparatus used in the method of the prior art 3. The apparatus shown in FIG. 7 differs from the apparatus shown in FIG. 6 only in that an annular high-frequency heating coil 22 is arranged so as to surround the heating furnace 16, and that a cylindrical susceptor 23 made of a high-melting-point element such as carbon, molybdenum or tantalum is provided coaxially with the heating furnace 16 near the inner surface of a portion of the heating furnace 16, where the high-frequency heating coil 22 is arranged. According to the apparatus shown in FIG. 7, the alloy material in the crucible 21 is heated and melted by the radiation heat of the susceptor 23 caused by the high-frequency heating coil 22.
The above-mentioned prior art 1 has the following problems: A rod-shaped alloy material having a chemical composition comprising Tb.sub.X Dy.sub.Y Fe.sub.Z is brittle in general. Therefore, an alloy rod having giant magnetostriction manufactured by applying a heat treatment to the rod-shaped alloy material having such a chemical composition is also brittle and easily cracks. In addition, the heat treatment applied to the rod-shaped material requires a long period of time, thus leading to a low manufacturing efficiency.
The above-mentioned prior art 2 has the following problems: When locally and sequentially melting the rod-shaped alloy material in the axial direction thereof and then locally and sequentially solidifying the resultant molten section of the alloy material, the molten section is held between the not yet melted alloy material and the melted and solidified alloy rod under the effect of surface tension of the molten section. However, the Tb.sub.X Dy.sub.Y Fe.sub.Z alloy in a molten state has only a small surface tension, with a high density. When a diameter of the alloy material is large, therefore, the molten section between the not yet melted alloy material and the melted and solidified alloy rod falls down in the form of drops, thus making it impossible to manufacture the alloy rod. According to the experience of the inventors, a diameter of the alloy rod capable of being stably manufactured in accordance with this method is 10 mm on the maximum even when adjusting the frequency and the output power of the high-frequency heating coil for melting the alloy material. There is at present a demand for an alloy rod having giant magnetostriction, having a large diameter of over 10 mm, for use as elements for an electric audio converter, a vibrator, an actuator and the like having a large output. However, such a large-diameter alloy rod having giant magnetostriction cannot be manufactured by the prior art 2.
The above-mentioned prior art 3 has the following problems: When moving the crucible 21 in the heating furance 16 to a level lower than the portion of the heating furnace 16, where the electric-resistance heater 17 or the high-frequency heating coil 22 is arranged, for cooling of the crucible 21 to solidify the resultant melt 24 of the alloy material in the crucible 21, heat of the melt 24 is radiated diagonally downward outside the crucible 21 and the heating furnace 16 as indicated by the arrows in FIGS. 6 and 7. As a result, the interface between the melt 24 of the alloy material and the solidified mass 24' of the alloy rod in the crucible 21 takes the form of a concave surface on the side of the solidified mass 24', resulting in an inappropriate temperature gradient of the solidified mass 24' near the interface. Consequently, the solidified mass 24' of the alloy rod cannot have a single-crystal structure or a unidirectional-solidification structure consistent with the axial line thereof. It is therefore difficult to manufacture an alloy rod having giant magnetostriction by the prior art 3.
Under such circumstances, there is a strong demand for the development of a method for manufacturing stably at a high efficiency an alloy rod having a large diameter of over 10 mm and giant magnetostriction without causing cracks, but such a method has not as yet been proposed.